This invention relates generally to radiation collimators, and more particularly, to leaf-type X-ray collimators for use in diagnostic medical imaging.
X-ray collimators are used in medical imaging applications to limit the field of an X-ray beam to a shape and size just sufficient to expose the area requiring diagnosis in a patient's body, and prevent unnecessary exposure of the surrounding area to X-rays. In other terms, a collimator helps to minimize the X-ray exposure and maximize the efficiency of X-ray dosage, to obtain optimum amount of pictorial data for diagnosis.
Generally, X-ray collimators provide a reduction in the field of an X-ray beam, by collimating the X-ray beam either to a substantial rectangular shape, a circular shape or a combination thereof, depending upon the configuration of the leaves or blades that block the X-rays for field reduction.
A typical configuration of an X-ray collimator that provides a rectangular collimation, includes at least a pair of planar blade members constructed of an X-ray attenuating material and arranged along the path of X-rays, which when moved to closer proximity in mutually opposing directions, block the X-rays, and thereby reduce the field of X-ray to a substantially rectangular shape for focusing on the area of a patient's body requiring diagnosis. However, the rectangular field shape encompasses a fairly large area of X-ray exposure as against the useful area of image and therefore results in low dosage efficiency.
The dosage efficiency “{acute over (η)}” is given by the relation:{acute over (η)}=Useful area of Image/Emitted area in same plane
A typical configuration of an X-ray collimator that provides a circular collimation includes a discrete set of discs constructed of an X-ray attenuating material and arranged in a circular fashion, along the path of X-rays. On actuation, the discs limit the field size of X-ray beam to variable diameters, thereby providing a discrete circular collimation, for focusing on an area of a patient's body, requiring diagnosis. Although the discrete circular field shape encompasses comparatively lesser area of X-ray exposure than the rectangular field shape, the drive mechanism for the discs is complicated in structure, and also there is no significant increase in the dosage efficiency.
Another known configuration of an X-ray collimator (also popularly used for collimating gamma radiation in nuclear medicine), that provides a circular collimation includes eight to sixteen leaves constructed of an X-ray attenuating material, and arranged in a “camera-iris” type configuration. On actuation, the leaves allow increase or decrease in diameter of the X-ray beam, thereby obtaining a fairly continuous circular collimation, for focusing on the area of a patient's body requiring diagnosis. Although this configuration provides an improved dosage efficiency and enables performing a nearly continuous circular (e.g. octagonal) collimation by limiting the field of X-rays to a substantially larger extent than the discrete collimation technique, the collimator is complicated in structure and also very expensive (although feasible for use in nuclear medicine due to high risks associated with gamma ray exposure) for use in an X-ray apparatus.
Yet another configuration of a circular collimator is disclosed in the European Patent Document EP 1 026 698 A2, published Oct. 8, 2000, applicant “Ein-Gal, Moshe”, which provides a novel revolving collimator system that can shape a radiation beam emanating from a radiation source with a plurality of mutually alignable collimators and pre-collimators. The collimators and pre-collimators are mounted on a plurality of revolving plates preferably stacked along a common axis. A control system with servomotors selectively rotates any one of the collimator plates, thereby aligning a plurality of collimators to form a path for collimating a radiation beam. This collimator, collimates and pre-collimates radiation beams over a wide range of diameter apertures suitable for virtually any kind of radiotherapy treatment plan. Although this system enables collimating the radiation beam to circular shape with different diameters, the system is much more complex as it makes use of selective and independent control mechanisms for each one of the collimator plates.
Yet another known configuration of a circular collimator includes a slidable leaf member having a collimating aperture therewithin, wherein the degree of sliding is proportional to the projected area of image exposure. Although this configuration adopts a simple mechanism, and allows continuous circular collimation, the dosage efficiency is not apparently significant.
Although these known collimators provide either a circular collimation, rectangular collimation or a combination thereof, none of the collimators provide (i) a simple configuration (ii) improved dosage efficiency (iii) efficient collimation and (iv) a cost effective solution for collimating X-rays, in terms of risk associated with X-ray exposure vis a vis the effort of treatment.